The present invention relates to semi-submersible column stabilized vessels such as drilling units of the type which remain afloat and at anchor or dynamically positioned by means of thrusters during drilling operations.
One approach to semi-submersible vessels has been to provide sufficient submerged structure to provide compensating weight so that the centre of gravity of the vessel is below the centre of buoyancy. Such structures are very large and costly to build and consequently the trend has been to construct semi-submersible column stabilized vessels, that is to say, vessels in which the centre of buoyancy is located beneath the centre of gravity and which, therefore, depend on positive metacentric height and righting arms for maintaining their upright configuration when afloat.
Recent disasters and near disasters with deep sea semi-submersible drilling rigs have caused the safety of certain hitherto accepted rig designs to be called into question. However, because drilling rigs are so functional in their form with their pairs of slender columns providing a small waterplane area it has been difficult for naval architects to realise how to improve the survival capability of the drilling units without interfering with their drilling function.
Two major economic factors govern the design of semi-submersible drilling units, the amount of Variable Deck Load (VDL, which is the weight which can be carried, typically about 1.5 m above the pipe racks) and the weather window (WW, which is the average number of days per year when the unit can perform drilling owing to sufficiently small wave induced motions). There exists a conflict in the requirements of large VDL and WW, since VDL is limited by stability requirements, and a large waterplane area, whilst being conducive to satisfactory stability, is decidedly detrimental to the motion characteristics.
As a result, drilling units are designed to perform drilling at drilling draught in moderate weather conditions and to have the capability to reduce draught and top deck load significantly and in a reasonably short period of time in order to withstand severe storm conditions. The procedures for this operation dictate that sufficient ballast and consumables should be discharged overboard in the process.
It follows that a significant improvement in semi-submersible design can be achieved if:
(a) VDL is increased without adverse affects upon WW, PA1 (b) the necessity of discharging ballast and consumables in order to change the mode of operation between the drilling and survival draught is reduced or eliminated. PA1 (a) adjacent the pontoon means close to the vessel transit waterplane during vessel transit; PA1 (b) adjacent and above the pontoon means and beneath the water during a drilling operation; and PA1 (c) at a survival waterplane position to increase pitching, heaving and rolling of the vessel with the waves during heavy weather conditions.
For the purpose of enhancing the VDL capability of semi-submersible drilling units, one or more of the following modifications are normally introduced: additional columns; blisters on the column to pontoon connections in order to improve transit stability; blisters on the columns at operating draught; and increase in pontoon displacement.
Addition of columns and/or blisters on the columns at operating draught adversely affects the forces exerted upon the vessel by waves and currents during operation and may involve a significant increase of constructions costs.
Increase in pontoon displacement does not overall affect stability if it does not involve a corresponding increase of the amount of ballast, and hence it does not provide efficient means of improving stability; besides, it can also be expensive. Blisters on the column to pontoon connections, although important for the transit condition, do not influence stability at the drilling and survival draughts.